1. Technical Field
The present invention generally relates to an active load circuit and, more particularly, to an active load circuit capable of being automatically connected to a dummy load to prevent a power supply from being in unstable conditions when there is no load or a very light load.
2. Description of Related Art
In response to the trend of energy saving and environmental protection, the power consumption of the central processing unit in a modern computer system is getting lower. When the computer system is in an idle mode, the power consumption is approximately equal to that in a no-load state. However, when the computer system is in a no-load state, the power conversion performance of its internal power supply is unstable and this may result in malfunction of the computer system.
More particularly, please refer to FIG. 1 and FIG. 2, where FIG. 1 is a block diagram of a conventional power supply and FIG. 2 shows the waveform of a power-good input signal of a conventional power supply. Generally, as shown in FIG. 1, the power supply 1 includes a main input circuit 11′, a transformer 12′, a main output circuit 13′ and a detection circuit 14′. The main input circuit 11′ includes an electromagnetic interference (EMI) filter circuit, a rectifier circuit and a power factor correction (PFC) circuit. The main output circuit 13′ includes an output filter circuit. The power supply 1 filters out the high-frequency noise in the AC power AC', rectifies the AC power AC' and reduces the harmonic current via the main input circuit 11′. Moreover, the power supply 1 is capable of smoothing output ripples via the main output circuit 13′ to provide a load coupled to the power supply 1 with an output voltage VOUT'.
Moreover, the detection circuit 14′ is disposed on the secondary side of the transformer 12′ to detect the input voltage of the secondary side and to correspondingly generate a power-good input (PGI) signal PGI'. The voltage level of the power-good input signal PGI' indicates whether the power supply 1 is abnormal. Practically, when the power supply 1 is with no load or a very light load, the power consumption on the secondary side is lower than the minimum energy transmitted from the primary side. Meanwhile, the primary side stops transmitting energy for a short time to stabilize the output voltage. However, during the short time the primary side stops transmitting energy, the input voltage on the secondary side falls, which may easily result in malfunction of the computer system due to a correspondingly lowered voltage level of the power-good input signal PGI' as shown in FIG. 2.
To overcome the foregoing problems, it has been reported to add a dummy load on the secondary side or a detection circuit on the primary side. However, the efficiency of the power supply 1 would be lowered so that it fails to meet the efficiency requirement that a power supply is more than 80% efficient at 20% rated load (80PLUS 20%) by Environmental Protection Agency (EPA) if a dummy load is added on the secondary side. Moreover, the manufacturing cost would be higher due to complicated circuitry if a detection circuit is disposed on the primary side.